This invention relates generally to the rendering of latent electrostatic visible using multiple colors of dry toner or developer and, more particularly, to a developer apparatus including structure for suppressing the development of the fringe fields of complementary tri-level images while developing acceptable line images, notwithstanding the presence of relatively high cleaning fields.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to from electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the a reas not struck by radiation.
This charge pattern in made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The concept of tri-level xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein, the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negatively and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it a cross the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development system is biased to about the b ackground voltage. Such biasing results in a developed image of improved color sharpness.
In tri-level xerography, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.ddp or V.sub.cad, see FIGS. 1a and 1b). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.c or V.sub.dad (typically 100v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad than V.sub.white (about 600v), and the DAD developer system is biased about 100v closer to V.sub.dad than V.sub.white (about 400v).
Various techniques have heretofore been employed to develop electrostatic images as illustrated by the following disclosures which may be relevant to certain aspects of the present invention.
As disclosed in U.S. Pat. No. 3,457,900, magnetic brushes have been designed to give fringe field or solid area development by adjusting the conductivity of the carrier. It is also stated therein that they can also be made to tone areas of less charge and clean areas of greater charge giving what is known in the art as a reverse development.
As discussed in U.S. Pat. No. 4,397,264 which relates to a conventional xerographic image development system, conductive magnetic brush (CMB) development and insulating magnetic brush (IMB) development systems suffer from limitations in their abilities to meet the full range of copy quality requirements. Specifically, insulating magnetic brush development systems have difficulty in using one developer roller to develop both lines and solid areas. In order to optimize solid area development with an insulating developer material, the spacing between the developer roller and photoconductive surface must be made quite small. However, low density fine line development occurs at a larger spacing to take advantage of the accuracy of fringe field development with insulating materials. This permits development with high cleaning fields so as to minimize background development.
As further discussed in U.S. Pat. No. 4,397,264 conductive magnetic brush development systems inherently fail to faithfully reproduce low density lines. Conductive developer materials are not sensitive to fringe fields. In order to achieve low density fine line development with conductive developer materials, the cleaning field must be relatively low. This produces relatively high background.
U.S. patent application Ser. No. 913,181 , now U.S. Pat. No. 4,761,668 filed on Sept. 29, 1986 Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus for minimizing the contamination of one dry toner or developer by another dry toner or developer used for rendering visible latent electrostatic images formed on a charge retentive surface such as a photoconductive imaging member. The apparatus causes the otherwise contaminating dry toner or developer to be attracted to the charges retentive surface in its inter-document and outboard areas. The dry toner or developer so attracted is subsequently removed from the imaging member at the cleaning station.
U.S. patent application Ser. No. 78,750, now U.S. Pat. No. 4,761,672 filed on July 28, 1987 in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus wherein undesirable transient development conditions that occur during start-up and shut-down in a tri-level xerographic system when the developer biases are either actuated or de-actuated are obviated by using a control strategy that relies on the exposure system to generate a spatial voltage ramp on the photoreceptor during machine start-up and shut-down. Furthermore, the development systems'bias supplies are programmed so that their bias voltages follow the photoreceptor voltage ramp at some predetermined offset voltage. This offset is chosen so that the cleaning field between any development roll and the photoreceptor is always within reasonable limits. As an alternative to synchronizing the exposure and developing characteristics, the charging of the photoreceptor can be varied in accordance with the change of developer bias voltage.
U.S. Pat. application Ser. No. 78,743 filed on July 28, 1987 in the name of Jerome May and assigned to the same assignee as the instant application which relates to tri-level printing discloses apparatus wherein undesirable transient development conditions that occur during start-up and shut-down in a tri-level xerographic system when the developer biases are either actuated or de-actuated are obviated by the provision of developer apparatuses having rolls which are adapted to be rotated in a predetermined direction for preventing developer contact with the imaging surface during periods of start-up and shut-down. rolls of a selected developer housing or housings can be rotated in the contact-prevention direction to permit use of the tri-level system to be utilized as a single color system or for the purpose of agitating developer in only one of the housings at a time to insure internal triboelectric equlibrium of the developer in that housing.
U.S. patent application Ser. No. 947,321, now U.S. Pat. No. 4,771,314 filed on Dec. 29, 1986 in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses printing apparatus for forming toner images in black and at least one highlighting color in a single pass of a charge retentive imaging surface through the processing areas, including a development station, of the printing apparatus. The development station includes a pair of developer housings each of which has supported therein a pair of magnetic brush development rolls which are electrically biased to provide electrostatic development and cleaning fields between the charge retentive surface and the developer rolls. The rolls are biased such that the development fields between the first rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the second rolls and such that the cleaning fields between the second rolls in each housing and the charge retentive surface are greater than those between the charge retentive surface and the first rolls.
U.S. patent application Ser. No. 95,486 filed on Aug. 31, 1987 in the name of Delmar Parker and assigned to the same assignee as the instant application which relates to tri-level printing discloses a magnetic brush developer apparatus comprising a plurality of developer housings each including a plurality of magnetic rolls associated therewith. The magnetic rolls disposed in a second developer housing are constructed such that the radial component of the magnetic force field produces a magnetically free development zone intermediate a charge retentive surface and the magnetic rolls. The developer is moved through the zone magnetically unconstrained and, therefore, subjects the image developed by the first developer housing to minimal disturbance. Also the developer is transported from one magnetic roll to the next. This apparatus provides an efficient means for developing the complementary half of a tri-level latent image while at the same time allowing the already developed first half to pass through the second housing with minimum image disturbance.
U.S. patent application Ser. No. 31,627 filed on March 3, 1987, and abandoned and filed as continuation application Ser. No. 220,408 on June 28, 1988 in the name of Parker et al and assigned to the same assignee as the instant application which relates to tri-level printing discloses an electronic printer employing tri-level xerography to superimpose two images with perfect registration during the single pass of a charge retentive member past the processing stations of the printer. One part of the composite image is formed using Magnetic Ink Character Recognition (MICR) toner, while the other part of the image is printed with less expensive black, or color toner. For example, the magnetically readable information on a check is printed with MICR toner and the rest of the check in color or in black toner that is not magnetically readable.
In tri-level xerography, the images comprise charged area images and discharged area images. Such images are commonly referred to as charged area development (CAD) images and discharged area development images, respectively. In a typical configuration where the charge retentive surface is uniformly charged negative, the CAD image is developed using a charged area development (CAD) system including a positive black toner with subsequent development of the discharged area using a discharged area development (DAD) system including a negative colored toner. When a CAD image or a background (V.sub.white) region moves past the DAD housing a reverse development or cleaning field is established between the image and the developer rolls of that housing. The magnitude of the field is determined by the difference between the voltage level of the CAD image after development which is approximately equal to the CAD bias voltage V.sub.bb (FIG. 1b), or the background, V.sub.white and the bias voltage on the discharged area development (DAD) system which is V.sub.cb. The field thus established tends to cause the negative toner to migrate away from the photoreceptor towards the developer rolls. Thus, when a fine line moves trough the DAD developer housing, particularly with its smallest dimension travelling in the process direction, the development field generated by the fine line doesn't have time to attract enough toner that has drifted away from the photoreceptor surface into the developer back to the charge retentive surface to adequately develop the DAD fine line image. Because of the toner's inertia, it takes it a finite time for the toner to move in response to a rapidly changing development field, and in the case of a fine line, perpendicular to the process direction, there may not be sufficient time if the toner has migrated too far into the developer. Thus, line images may be improperly developed. This phenomenon is known as a developer history effect, which in this case is manifest as an underdeveloped fine line.
The shortcomings of CMB and IMB development systems discussed above with respect to conventional xerography have, heretofore, been present in highlight color xerography as well. In fact, the problem of not being to able to develop low density fine lines with CMB developer in the presence of relatively high cleaning fields has resulted in the wide use of IMB developer. However, in a tri-level highlight color system the use of IMB developer has been found to be unacceptable. Its use results in the development of fringe fields in a color different from the rest of the image. Thus, for example, in a system that uses black and red developers, the black images would have a red border around them while red images would have a black border around them.
The development of such fringe fields is caused by the reverse development or cleaning fields established between the developer biases and a complementary image (either developed or latent) on the charge retentive surface. The colored around the black image results from the field established due to the difference (V.sub.bb -V.sub.cb, see FIG. 1b) between the developer biases as the black images passes through the red developer housing while the black border around the red image results from the field established due to the difference (V.sub.bb -V.sub.c) between the bias on the black developer housing and the voltage level of the red latent image on the charge retentive surface as that image passes through the black developer housing.
Since the use of IMB developer has been found to be unacceptable in a tri-level highlight system for the reason noted, and since CM developer as disclosed in the prior art cannot develop line images in the presence of relatively high cleaning fields it would appear that tri-level highlight color imaging in a single pass is not viable.